1. Field of the Invention
This invention relates to electrically driven mine shuttle cars and more particularly to the mechanism carried thereby for reeling in and unwinding cables through which electrical current is delivered to the vehicles.
2. Description of the Prior Art
In many applications there are requirements for self-propelled vehicles which utilize a remote power source such as an electric generator to power the vehicle. Normally these vehicles have cable reels mounted thereon for winding in and unwinding the flexible electric cable. Such vehicles as mobile compressors, mine shuttle cars, continuous mining machines, mine locomotives, have cable reel drive and control apparatus for the reel which include a reversible hydraulic motor, a hydraulic pump and a control valve.
With reeling devices of the type mentioned, it is important that they shall be usable not only to wind up the cable when moving toward the point of connection of the latter to the source of power and also to permit the unwinding of such cable when moving away from such connection point, but also when they are moved in courses or paths which will occasion a need for automatically shifting from a winding in to an unwinding, or from an unwinding to a winding in operation that they shall automatically affect such shifting.
It is also important that when the unwinding of cable ceases, there shall be an automatic repositioning of parts so that there may be no possibility of the cables being run over, and also to forestall the accumulation of slack in the cable which might interfere with its smooth winding.
It is therefore important to maintain some tension on the cable even during times when it is being pulled off the reel.
There have been a variety of control devices which control the torque imparted to the cable reel by its drive motor to accomplish the objectives enumerated. Such devices can be seen in U.S. Pat. Nos. 2,589,217; 2,707,598; 2,639,101; 2,670,153; 3,380,545 and 4,108,264.
In general, these prior art devices control the torque produced by the cable reel drive motor at two discrete levels. The first relatively low torque level is utilized when the machine is moving in a direction which tends to unwind the cabel. This torque produces enough force in the cable to take up the slack and keep the cable in the vicinity of the machine, off the ground. A second torque level is utilized as the vehicle moved in the direction tending to wind in the cable. The higher torque is necessary to insure that even with the cable reel drum almost full with cable that there would be sufficient force on the cable to lift it off the ground thereby ensuring that the vehicle does not run over its own cable. In addition, a greater torque is required to wind in the cable since, unlike when the machine is moving away from the power source, the movement of the vehicle does not facilitate the winding in and reel must be driven by the hydraulic system to wind in the cable.
U.S. Pat. No. 4,108,264 takes a different approach to the problem by controlling the output of the drive motor in response to the slack angle of the cable to the vertical.
It has been found that with the high and low torque control devices that the cable is under very high tension when most of the cable is off the reel and particularly when the vehicle travels in a direction in which the cable must be rewound on the reel. Since the force in the cable is equal to the torque divided by the radial distance from the cable reel drum axis, the tension in the cable decreases in proportion to the amount of cable on the reel as the rewinding progresses.
The invention disclosed herein is intended to maintain the tension on the cable at a constant level by varying motor torque in response to the amount of cable on the reel. This allows a minimum torque to be applied to the cable reel by the drive motor when there is relatively little cable on the reel and a much higher torque when the cable reel is almost full. The tension is always maintained within a predetermined limit either in the high or low torque position which limit and the lack of cyclic forces on the cable causes minimum wear on the cable.